(1) Field of the Invention
The present invention relates to a communication system in which a multicall (multi-call) communication mode is supported, and more particularly, the present invention relates to a mobile-service switching center, a base station controller, a multicall mode supporting terminal and a method of changing number of calls in multicall communication mode which are suitable for use in a multi-session control system for an interwork between a next-generation network and the existing network.
(2) Description of Related Art
In the existing ordinary telephone network and a mobile-service communication network, when one of a communication service in a voice signal communication, a packet communication or the like is realized, one mobile terminal occupies one line of communication traffic in the network.
FIG. 28 is an overall arrangement of a mobile-service communication system. As shown in FIG. 28, a mobile-service communication system 99 includes a mobile station (MS) 100 such as a portable telephone terminal or the like, a base station (BSS) 101a, 101b, a switching center 102a, 102b, a home location register (hereinafter referred to as HLR) 103, the public switched telephone network (hereinafter referred to as PSTN) 104a and the internet 104b. 
The switching center 102a, 102b is known as a mobile-service switching center. The mobile-service switching center includes therein a visitor location register (hereinafter referred to as VLR). Thus, the mobile-service switching center is denoted as MSC/VLR. In this case, the VLR is a unit for temporarily holding generated data of a subscriber.
The HLR 103 is utilized for holding and managing data of the subscriber such as of a telephone number of the mobile station 100, registration of location of the subscriber, and authorization of the subscriber. These components of the communication system work in a cooperative manner so that a subscriber can be registered in his or her location wherever the subscriber with the mobile station is located. Further, owing to the cooperative work of these components, a call made from the mobile station 100 can be effectively found by the network.
In this way, the mobile station 100 can communicate with its opponent party with a voice signal through the base station 101b and the public switched telephone network 104a. Further, the mobile station 100 can carry out packet communication through the base station 101b and the internet 104b. 
Incidentally, as a communication system for the next-generation, a standard of IMT-2000 as the third generation mobile communication system includes a regulation of multicall mode communication in which a single mobile terminal can settle a plurality of communication channels at a time. Further, the standard of IMT-2000 also includes a regulation of multiconnection communication mode in which a single call is established by settling a plurality of communication links. According to the regulations of the standard of IMT-2000, a subscriber can transmit or receive text data while enjoying telephone conversation. Alternatively, the subscriber can transmit and receive an e-mail while transmitting video image to other person. By the way, the multisession communication mode (multicall communication mode) conceptually includes VoIP (Voice Over IP) in which voice data is converted into an IP packet and sent to its destination through the internet.
As one of available communication service scheme for connecting a single mobile terminal to a plurality of calls, there has been proposed a so-called “catch-phone” (service name). This communication service scheme is different from the multicall communication mode. The difference between the multisession and the communication service scheme named catch-phone will hereinafter be described with reference to FIGS. 29 and 30.
FIG. 29 is a diagram showing an example of connection sequence of the catch phone. As shown in FIG. 29, a subscriber, base station controller, a switching center, and subscribers as connection destinations 1 and 2 take statuses of S1, S4, S5, S7, S8 and exchange messages of S2, S3, S6 among them. Initially, when the subscriber and the connection destination 1 occupy a communication line (status attached with notation S1), the other subscriber as a connection destination 2 makes a call to the subscriber (status S2: calling request). Thereafter, the subscriber transmits a message of connection hold request S3 regarding the connection destination 1 to the switching center. In this way, the call from the connection destination 1 is brought into a hold mode (status S4) while the subscriber goes to a communication mode with the connection destination 2 (status S5). Further, the subscriber transmits message S6 to the switching center so that the connection destination 2 is brought into the hold mode (status S7) while the subscriber goes to the communication mode with the connection destination 1 (status S8).
As described above, according to the communication service scheme of the catch phone, since a call is temporarily held at the switching center, the communication line settled between the mobile terminal and the switching center is not more than a single line.
FIG. 30 is a diagram showing an example of connection sequence when the multisession communication mode is taken place in the communication service network. As shown in FIG. 30, the connection sequence includes three statuses T1 to T3 and Message T2. If a connection destination 2 makes a call to the subscriber with Calling Request Message T2 when the subscriber and the connection destination 1 of FIG. 30 occupy a communication line (status T1), then the subscriber goes to a communication mode with the connection destination 2 (status T4) while the communication line with the connection destination 1 is maintained (status T3).
As described above, according to the multisession communication mode, a plural number of communication lines can be settled between the mobile terminal and the switching center.
Further, according to the third generation of mobile communication system, it is essentially requested to establish a technology of handover between a network based on the IMT-2000 standard and the existing network. The term handover section that, for example, when a mobile station in a vehicle is moving from one cell to another, a new communication traffic is established to continue the call with the mobile terminal. If any proprietary network employs a communication system based on the IMT-2000 standard, how the number of calls busy in communication should be handled is a critical problem.
Japanese Patent Laid-Open (Kokai) HEI 11-261717 (hereinafter referred to as a prior-art reference) discloses a technology relating to an improvement in transmission work efficiency in a communication system when image data (photograph data) photographed at a local place is once taken into a mobile terminal and sent to an office or the like by means of a communication equipment such as a mobile terminal and so on.
However, according to the above conventional technology, when the multicall communication mode is effected, and handover process is requested or a congesting state of the network is fluctuated, causing that only a single call becomes allowable to continue, then the network side arbitrarily selects a call to be continued and other calls are discarded. Thus, there is no allowance for the subscriber to select a call to be continued.
Therefore, if the network is placed under a state that only a single call can be supported and the switching center receives a call connection request under the congesting state, the switching center disconnects a call (if present) with a priority lower than the priority of the call corresponding to the call connection request, and the switching center selects a call of a relatively high priority from newly requested calls to provide a connection service. Accordingly, if there is no call with a low priority to be replaced with the newly requested call, the switching center has no choice other than to refuse the new connection request.
Further, according to the conventional technology, although emergency calls such as a police call, a fire station call, an ambulance call and soon are managed in terms of priority at every subscriber, a plurality of potential calls made by each subscriber were not managed in terms of priority. Furthermore, since the multicall communication mode as a communication service scheme is still not widely deployed, the matter relating to the multicall communication mode has not been sufficiently discussed.
In addition, the above-introduced prior art reference discloses merely a technology in which two sets of communication equipment such as a mobile terminal and a PHS (Personal Handy-phone System) are connected to each other and made allowable to communicate at a time, whereby data can be transmitted to a single destination office by using two channels of network at a time. Accordingly, the technology of the reference cannot support the multicall communication mode, and hence the technology does not support a function for restoring the number of calls that were once obliged to reduce due to a network problem or the like.